1. Field of the Invention
The present invention relates to an exhaust muffler, and more particularly to a manufacturing technology of an exhaust muffler for use in motor vehicles such as motorcycles, motorcars or the like.
2. Description of the Prior Art
Generally, as an exhaust muffler for use in motor vehicles, there is known a type of exhaust muffler which has a joint construction as seen in FIG. 11, for example. The exhaust muffler 51 is provided with an outer cylindrical body 52 of rolled steel plate, a pair of end plates 53, 54 of rolled steel plate each joined with a front, and a rear ends of the outer cylindrical body 52, and an internally installed unit 55 accommodated within the outer cylindrical body 52. The internally installed unit 55 comprises porous plates 5G, 57, 58 arranged on and spaced apart from the inside surfaces of the outer cylindrical body 52 and each of the end plates 53, 54, partition plates 59, 60, 61 for partitioning the longitudinally extending internal space of the outer cylindrical body 52 into several chambers, and a plurality of communication pipes carried on each of the partition plates 59, 60, 61 and extending into and between each of the partitioned chambers. An acoustic absorption material 62 of glass wool or the like is inserted into clearances formed between the outer cylindrical body 52 and the porous plate 56 and between each of the end plates 53, 54 and each of the porous plates 57,58. When assembling this type of muffler 51, the outer cylindrical body 52, on a predetermined region of which a plurality of plug holes are provided preparatory to welding, is rolled into a cylindrical shape while the internally installed unit 55 is formed into an assembly by winding the porous plate 56 around the partition plates 59, 60, 61 which carry the communication pipes thereon. The internally installed unit 55 is inserted into the outer cylindrical body 52 in such a way that the plug holes for the welding of the outer cylindrical body 52 are positioned on the joint portions of each of the partition plates 59, 60, 61. Then, the outer cylindrical body 52 and the internally installed unit 55 are partially fixed to each other, prior to being completely joined, by spot welding, for example, and next completely joined with each other through the plug holes of the outer cylindrical body 52 by MIG (metal inert gas) welding.
The pair of front and rear end plates 53, 54 are firstly engaged with each of the corresponding porous plates 57, 58 to form the respective assembled units. The assembled units are joined by welding with the front and rear ends of the outer cylindrical body 52 so that the exhaust muffler is completely assembled.
In this manufacturing process, however, many steps for welding and others are required, and the surface treated regions of the outer cylindrical body 52 and the end plates 53, 54 each made of rolled steel plate are thermally affected at the time of welding and suffer surface destruction to a large extent which requires an after-treatment such is a full surface coating. Also, it, is necessary to cope with the thermal strain due to welding. Further, since the welding between the partition plates 59, 60, 61 and the outer cylindrical body 52 is done by the spot, welding through the plug holes of the latter, it is difficult to have the partitioned chambers kept in a sufficient air-tight condition.
On the other hand, there is known an exhaust muffler 51 with an outer cylindrical body of double or multiple rolled construction. The exhaust. muffler 51 of this type has better characteristics than the same with a single rolled outer cylindrical body 52 with respect to noise absorption, and thus may be made compact in size, while enabling the internally installed unit 55 to be simplified. This type of multiple-rolled outer cylindrical body is made by rolling up a sheet of steel plate material into a double- or multiple-layered cylindrical shape through roll forming or other processes. The internally installed unit 55 is thereafter inserted into and coupled with the thus-formed outer cylindrical body 52. The multiple-rolled outer cylindrical body of this type, however, is not capable of squeezing or tightly holding the internally installed unit 55 with the outer cylindrical body 52 when they are joined with each other. The joining method between the internally installed unit 55 and the outer cylindrical body therefore becomes complicated, whereby the manufacturing process takes a long time and an air-tight, condition of the partitioned chambers may not, be maintained.
Furthermore, when the outer cylindrical body 52 is formed in a multiple-layered construction from a single sheet of steel plate material, there remain some difficulties in obtaining a reduction in weight, a diversification of design in external appearance, a reduction in cost of material and the like.